Frequency hopping is often employed in cellular, radio telecommunications systems, such as the Global System for Mobile Communication (GSM), to improve system performance. In general, frequency hopping improves system performance by introducing frequency diversity and interference diversity, as will be explained in detail below. Frequency hopping is a well-known technique.
In a radio telecommunications system, frequency diversity is achieved by transmitting each radio telecommunications signal on a sequence of frequencies over time. Each radio signal is transmitted over a sequence of frequencies because radio signals are often subject to amplitude variations called Rayleigh fading. However, Rayleigh fading generally affects radio signals carried on some frequencies more so than other frequencies. Thus, transmitting a radio telecommunications signal over a sequence of different frequencies increases the likelihood that the signal will be received correctly, as it is unlikely that Rayleigh fading will significantly impact each and every frequency over which the radio telecommunications signal is being transmitted. Accordingly, signal quality is improved and overall system performance is enhanced.
On the other hand, Interference diversity works as follows. In addition to fading, a radio signal is often subject to varying degrees of interference caused by traffic on the same frequency (i.e., co-channel interference) and traffic on an adjacent frequency (i.e., adjacent channel interference). If co-channel and/or adjacent channel interference is substantial, the signal quality associated with the radio signal may be severely impacted, so much so, that the connection may be dropped. In theory, frequency hopping, through the introduction of interference diversity, spreads the co-channel and adjacent channel interference amongst numerous end-users, such that the co-channel and adjacent channel interference experienced by a particular end-user is diversified. The overall effect is to raise the signal quality across the network, thereby improving overall system performance.
While frequency hopping improves system performance by improving signal quality, frequency reuse is designed to improve system performance by increasing system capacity. More specifically, frequency reuse permits two or more cells to simultaneously use the same frequency, or group of frequencies, so long as the distance (i.e., the "reuse distance") between the two cells is sufficient to minimize any co-channel interference that might otherwise have an adverse affect on signal quality. However, as the demand for cellular service increases, reuse distances are likely to decrease. And, as reuse distances decrease, co-channel interference is likely to increase.
To limit co-channel interference, fractional loading may be employed. In a cellular network that employs fractional loading, the number of transceivers installed in each cell is less than the number of frequencies allocated to each cell. With synthesizer frequency hopping, each transceiver hops on all the allocated frequencies, but at any instant in time, the number of frequencies being transmitted by any cell is at most equal to the number of installed transceivers. Since each cell in the conventional, unsynchronized network is typically given a different frequency hopping sequence, at any given instant, potentially interfering cells are unlikely to be emitting exactly the same frequencies. The average interference in the network is thereby reduced.
Another technique that is employed to improve overall system performance is known as base station synchronization. In a Time Division Multiple Access (TDMA) system such as GSM, base stations may be synchronized or unsynchronized with respect to each other. In an unsynchronized system, each base station independently transmits and receives radio communication bursts. Consequently, a radio communication burst associated with one base station will overlap in the time domain with two sequential radio communication bursts associated with each of a number of proximally located base stations. In a synchronous system, base stations, typically in groups of three or more, transmit and receive radio communication bursts in a synchronized manner with respect to each other. Thus, the radio communication bursts transmitted by one base station are aligned in the time domain with the radio communication bursts transmitted by the other base stations affiliated with the group of synchronized base stations. Likewise, radio communication bursts received by one base station are aligned in the time domain with the radio communication bursts received by the other base stations affiliated with the group. In general, synchronization provides an element of control, whereby a system or network operator is better able to manage the level of co-channel and adjacent channel interference through careful allocation of frequencies and frequency offsets. However, in order to achieve this additional control, in a system that employs frequency hopping techniques, it is necessary for all synchronized cells to follow the same frequency hopping sequence (i.e., a reference frequency hopping sequence). By providing a mechanism to better control interference through proper and prudent frequency and frequency offset allocation, system performance may be significantly improved. Frequency offset management is particularly useful if fractional loading is employed, as will be discussed further below.
FIG. 1 illustrates a subset of cells A, B, C, A', B' and C' associated with a synchronized, cellular radio telecommunications system 100. System 100, as shown, employs a frequency reuse plan, and more particularly, a one-reuse plan, as all frequencies are potentially used by each cell. Thus, in accordance with the example illustrated in FIG. 1, a mobile station operating in cell A may simultaneously operate over the same frequency as a mobile station operating in cell A'.
In addition, system 100 employs fractional loading and offset management. In accordance with fractional loading and offset management, the group of cells comprising cells A, B and C is allocated a common set of frequencies. If, for example, twelve frequencies 1-12 are allocated, there are inherently twelve frequency offsets 0-11. Then, in any one cell, a fraction of the twelve frequency offsets is assigned. For instance, cell A may be assigned frequency offsets 1, 4, 7, 10, where it will be understood that it is preferred to have at least a few frequency units between each frequency offset assigned to each cell so as to mitigate adjacent channel interference.
In accordance with conventional frequency hopping techniques, a reference frequency hopping sequence is established for the entire system. A mobile station, at handover or call set-up, is then assigned an available frequency offset associated with the cell in which the mobile station is operating. The mobile station hops through a sequence of frequencies that are, over time, offset from the reference frequency hopping sequence by a fixed amount that is equal to its assigned frequency offset. In accordance with the GSM standard, each frequency offset is referred to as a Mobile Allocation Index Offset (i.e., MAIO).
To better illustrate conventional frequency hopping, FIG. 2 depicts an exemplary, reference frequency hopping sequence, over a time period t.sup.1 -t.sub.10, for the telecommunications system 100. As shown, the reference frequency hopping sequence over the time period t.sub.1 -t.sub.10 is [9,5,11,1,3,9,12,10,7,8]. If a first mobile station operating in cell B is, for example, assigned frequency offset zero, the first mobile station will hop through the sequence [9,5,11,1,3,9,12,10,7,8] over the time period t.sub.1 -t.sub.10. If a second mobile station operating in cell A is assigned frequency offset seven, the second mobile station will hop through the sequence [4,12,6,8,10,4,7,5,2,3] over the time period t.sub.1 -t.sub.10. It is important to reiterate that in accordance with conventional frequency hopping techniques, the frequency offset assigned to each mobile station remains constant. Thus, the frequency offset between the first mobile station operating in cell B and the second mobile station operating in cell A remains fixed; in this case, it remains fixed at seven.
FIGS. 3A-3C illustrate, more clearly, conventional frequency hopping as it applies to several mobile stations operating in telecommunications system 100, where the reference frequency hopping sequence illustrated in FIG. 2 is assumed. More specifically, FIG. 3A illustrates three exemplary mobile stations operating in cell A, where each of these three mobile stations is assigned a frequency offset (i.e., a MAIO) of 4, 10 and 1 respectively. Accordingly, the mobile station represented by the symbol ".largecircle." is assigned a frequency offset of 4. Accordingly, this mobile station follows the frequency hopping sequence [1,9,3,5,7,1,4,2,11,12] over the time period t.sub.1 -t.sub.10. The mobile station represented by the symbol ".quadrature." is assigned a frequency offset of 10. Accordingly, it follows the frequency hopping sequence [7,3,9,11,1,7,10,8,5,6] over the time period t.sub.1 -t.sub.10. The mobile station represented by the symbol ".star." is assigned a frequency offset of 1. Thus, it follows the frequency hopping sequence [10,6,12,2,4,10,1,11,8,9] over the time period t.sub.1 -t.sub.10.
FIG. 3B illustrates one exemplary mobile station operating in cell C, wherein the one exemplary mobile station is represented by the symbol ".tangle-solidup.", and wherein this one exemplary mobile station is assigned a frequency offset 2. Accordingly, the mobile station follows the frequency hopping sequence [11,7,1,3,5,11,2,12,9,10] over the time period t.sub.1 -t.sub.10.
FIG. 3C illustrates two exemplary mobile stations operating in cell A', wherein these two mobile stations are represented by the symbols "X" and ".circle-solid.", and wherein the two mobile stations are assigned a frequency offset of 10 and 1 respectively. Thus, over the time period t.sub.1 -t.sub.10, the mobile station represented by the symbol "X" follows the frequency hopping sequence [7,3,9,11,1,7,10,8,5,6], just as the mobile station represented by the symbol ".quadrature." operating in cell A. Similarly, the mobile station represented by the symbol ".circle-solid." follows the frequency hopping sequence [10,6,12,2,4,10,1,11,8,9], just as the mobile station represented by the symbol ".star." operating in cell A.
As explained above, frequency reuse and fractional loading may limit interference diversity in systems such as telecommunications system 100, and in turn, the efficacy of conventional frequency hopping. This is best illustrated in FIGS. 3D-3F, wherein FIG. 3D, for example, illustrates the frequency offset, over the time period t.sub.1 -t.sub.10, between the three mobile stations operating in cell A. As shown by graph R.sub.(.star.,.largecircle.), there is no interference diversity between the mobile station represented by the symbol ".star." and the mobile station represented by the symbol ".largecircle.", as the frequency offset remains fixed at three frequency units. Likewise, graph S.sub.(.largecircle.,.quadrature.) indicates that there is no interference diversity between the mobile station represented by the symbol ".largecircle." and the mobile station represented by the symbol ".quadrature.", as the frequency offset remains fixed at six frequency units. Finally, as shown by graph T.sub.(.quadrature.,.star.), there is no interference diversity between the mobile station represented by the symbol ".quadrature." and the mobile station represented by the symbol ".star.", as the frequency offset remains fixed at 9 frequency units. However, despite the fact that graphs R.sub.(.star.,.largecircle.), S.sub.(.largecircle.,.quadrature.) and T.sub.(.quadrature.,.star.) indicate that there is no interference diversity, there is no co-channel interference between these mobile stations, as the mobile stations are never simultaneously operating over the same frequency. Nor is there any adjacent channel interference between the mobile stations, as the mobile stations, by virtue of the frequency offsets assigned to cell A, are never operating on adjacent frequency channels. Therefore, the lack of interference diversity between the mobile stations operating in cell A is not likely to result in any signal degradation.
The problems associated with the lack of interference diversity, and in particular, adjacent channel interference diversity, becomes more evident in FIG. 3E. FIG. 3E illustrates the frequency offset between the one exemplary mobile station operating in cell C, represented by the symbol ".tangle-solidup.", and the mobile station operating in cell A, represented by the symbol ".star.", over the time period t.sub.1 -t.sub.10. Similar to the graphs presented in FIG. 3D, the graph U.sub.(.tangle-solidup.,.star.) in FIG. 3E indicates that there is no interference diversity between the mobile station represented by the symbol ".tangle-solidup." and the mobile station represented by the symbol ".star.". More importantly, the graph U.sub.(.tangle-solidup.,.star.) indicates that there is no adjacent channel interference diversity between these mobile stations, as the frequency offset between the two mobile stations, over time, remains fixed at only one frequency unit. Regardless, this lack of adjacent channel interference diversity may not be problematic unless the offending mobile station is operating at or near the cell periphery, and/or the power level associated with the offending mobile station is significantly strong.
Although the lack of adjacent channel interference diversity associated with conventional frequency hopping can be a problem, as shown in FIG. 3E, the primary problem associated with conventional frequency hopping techniques is the lack of co-channel interference diversity, as illustrated in FIG. 3F. FIG. 3F illustrates the frequency offset between the mobile station represented by the symbol ".quadrature.", operating in cell A, and the mobile station represented by the symbol "X", operating in cell A', over the time period t.sub.1 -t.sub.10. FIG. 3F also illustrates the frequency offset between the mobile station represented by the symbol ".star.", operating in cell A, and the two mobile station represented by the symbol ".circle-solid.", operating in cell A'. As indicated by the graph T.sub.(.quadrature.,X),(.star.,.circle-solid.), there is no interference diversity, and in particular, no co-channel interference diversity, between the mobile stations, as the frequency offset between the mobile stations, represented by ".quadrature." and "X", and the frequency offset between the mobile stations represented by ".star." and ".circle-solid." are fixed at zero (0) frequency units. As one skilled in the art will readily appreciate, the likelihood that these mobile stations will serve as a source of severe co-channel interference with respect to each other is relatively high. Moreover, this likelihood increases as the power levels associated with the mobile stations increase, and/or the reuse distance separating cells A and A' decreases.
Given the fact that reuse distances are likely to decrease in time, as the demand for cellular services continues to increase, and given the fact that conventional frequency hopping techniques cannot, as described above, fully exploit the potential of synchronization due to inadequate co-channel interference diversity, and to a lesser extent, adjacent channel interference diversity, it is of particular interest to provide a frequency hopping technique that maximizes interference diversity, particularly in synchronized, cellular, radio telecommunications systems.